Tanning



Patented Mar. 17, 1942 Joseph S. Kirk, Cleveland, Ohio, assignor to E. I.

du Pont dc Nelnoul's a Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 4, 1941,

Serial No. 396,565

13 Claims.

This invention relates to the production of leather, and it is more particularly directed to compositions and processes for the tanning of skins with silicic acid sols containing a hydrogen bonding donor compound.

Silicic acid sols have been proposed as tanning agents but their use has been none to successful. It has been proposed to add various metal compounds such as aluminum or iron salts, but the modification has not led to satisfactory results. Tanning agents including silicic acid sols show so great an aflinity for skin substance that overtanning usually occurs. The leather produced is harsh, stiff, and of a generally unsatisfactory character. Changes occur in leather tanned with silicic acid sols according to the processes proposed in the art and the leather soon becomes even stiffer, sometimes to the point of brittle- BOSS.

It hasfurther been proposed tomodify so-called silica sol tanning agents by the inclusion of varinus organic compounds such as lactic acid and sulflte cellulose waste, but "the addition of such modifying agents has not effected sumcient improvement in the tannages to make them satisfactory. The leather produced is still ofpoor character and has only a poor quality compared with standard vegetable and chrome tans which have found acceptance in the trade.

It is an object of this invention to provide novel tanning processes and compositions. It is a further object to provide tanning compositions and processes which produce a good quality of leather at relatively low cost. It is a further object of this invention to provide compositions and processes employing silicic acid sols in the tanning of skins to produce leather of good character. It is a still further object to provide processes and compositions which permit rapid, simple, and easily-controlled tanning of skins. It is a still further object of this invention to improve upon prior art processes employing silicic acid sols for the tanning of skins. It is a still further object of this invention to produce white leather. Still further objects will become apparent hereinafter.

The foregoing and other objects of this inven tion are attained by the use of a water-soluble hydrogen bonding agent with a silicic acid sol. By the inclusion of a hydrogen bonding donor compound in silicic acid tann ns agents there are obtained tannages which are easily used and which are economical. Leather produced according to the preferred processes of this invenby lowering the pH of a solution of a soluble silicate. when first formed the sols probably contain considerable amounts of silicic acid but silicic acid is probably present as such only for a relatively short time and then conversion to silica begins. Most silicic'acid sols are, as is well-known, to an extent unstable and after a period of time v rying from a few hours and upwards change over to a silica gel.

As has been the practice heretofore, silica sols should .be used as tanning solutions before gelling occurs and before they become too astringent. It will generally be preferable to use'these silicic acid sols shortly after they are made up even though sols modified with hydrogen bonding donor compounds according to this invention are of increased stability.

Silicic acid sols for use as tanning agents according to this invention may be prepared in various other ways already known to the art. It will ordinarily be desirable to start with a soluble silicate such as sodium or potassium silicate and reduce the pH- of the silicate by adding it to an acidic material. Any acidic material may. be used though as will bepointed out hereinafter certain acidic metal salts themselves have an effect in the resulting product. There may be used for reducing the pH of a silicate such acidic materials as sulfuric acid, sulfamic acid, hydrochloric acid, nitric acid, thionic acids, acetic acid, lactic acid, and other similar inorganic and organic acids. Acidulous salts may similarly be used and there may be employed for instance, sodium bisulfate, monosodium phosphate, acid tartrates, zinc chloride, titanium sulfate, aluminum sulfate, and chromium sulfate. The amount of acid or acidic material will ordinarily be 'such as to result in a pH of about 1.0 to5.'0.

It will ordinarily be desirable to reduce the DH of a silicic acid sol to about 1.5 to 3.5. Still more specifically as will be pointed out hereinafter, it will be preferred to reduce the pH to about 2.0 to 3.0. It will be understood that while any acidulous material may be used to increase of two or more acidulous materials may be used as seems desirable.

There may be included in the silicic acid sols various metal compounds. If an acidulous metal compound is selected it may serve to supply a part or all of the acidity needed to produce the sol. There may be included in silicic acid sols according to this invention such metal cations as aluminum, chromium, iron, zinc, and titanium. These metals may be" added in the form of any of their soluble compounds and there may be used, for instance, aluminum sulfate, aluminum formate, aluminum lactate, aluminum chloride, chromium sulfate, titanium sulfate, and iron sulfate.

The preferred tanning compositions according to the present invention are comprised of silicic acid sols containing a soluble aluminum compound. Aluminum is preferably introduced into a silicic acid sol in the form of aluminum sulfate, due consideration being given to the acidity of this compound so that the final pH will fall within the range desired. The amount of aluminum introduced should be such that the A12O31S1O2 ratio lies between the limits of 1:2 to 1:4 parts by weight. With much smaller ratios than 1:2 excessive puckering of the leather takes place and above about the 1:4 ratio the leather tends to become fiat tinney. The ratio of Al:O::SiO:- can be obtained, as has been indicated above, by the use of various silicates and aluminum compounds with various acidulous materials as required.

The concentration of silicic acid sols may be selected as in prior processes with-this sol. Sols made up for use may contain about four per cent S102 and two per cent A1203 by weight. I

have used 6 per cent SiOz and 3 per cent A1203.

Upon dilution to make a tanning bath it will seldom be found desirable to have an $10: content in excess of about two per cent.

According to'the present invention silicic acid tanning agents are modified by the inclusion of a hydrogen bonding donor compound. Hydrogen bonding is -'a.- concept advanced in recent years to explainfcertain abnormalities in the chemical and physical behavior Ofmixtu'res -of compounds one .of whichcontains hydrogen attached to a strongly-"negative"radical .and the other an atom capable of donating a" pair of electrons to form-a; directional or .co-ordination bond.

Sincethe bond i's--formed by the donation of an properties of the mixture'indicate that a type of chemical compound is formed. These mixtures,

for instance, exhibit anabnormal vapor pres-,-

sure lowering, that is abnormal deviation from Raoults law. There is further-pbserved'abnormal heats of mixing andx'dbnormal deviation in viscosity and freezing pointlowering. I

I have found 1 that when silicic .acid

unexpected from.-a"consideration of the characacteristicsfoff the materials mixed. Apparently I mixed -with an organicjhydrogen bonding, .donor compound it exhibits characteristics unexplainable upon-the basis of-ordinaryfchemical reaction and smears It will be understood that in advancing the theory of hydrogen bonding to explain the unexpected results of the present invention I do not intend to be limited or restricted by this theory.

6 The theory may or may not be correct and for purposes of the present invention it is of importance largely because whatever the reason, the type of compounds known to be organic hydrogen bonding donors profoundly modify silicic acid 10 sols and gels.

It will be seen that these hydrogen bonding I donor compounds are of the type of materials which might be designated as weak organic bases. According to this theory the materials involved act as bases with silicic acid which is quite a strong acid. These bases are of such weakness that they will not raise the pH of a silicic acid solution above 7. According to this theory one may therefore describe the agents added to silicic acid sols according to the present invention as water-soluble, organic bases which will not raise the pH of silicic acid above 7. It is again observed that while the terminology of either the hydrogen bonding theory or the weak base 35 theory may advantageously be used in describing the present invention to those skilled in the art in commonly understood language applicant does not consider the adoption of either theory as bearing upon the invention save as it gives a convenient system of terminology.

While any hydrogen bonding donor compound (or weak base) may be used with silicic acid according to the present invention it will generally be found desirable to use any water-soluble hydragon-bonding donor compound selected from the group consisting of ethers, amides, alcohols, and ketones.

Silicic acidtanning agents prepared according to this invention are used in aqueous solution and hydrogen bonding donor compounds used should accordingly be water-soluble at least to the slight extent required to permit formation of a coordination complex with silicic acid. Preferably, the donor compound should be water-soluble. The structure of such materials as NN'-diphcnyl urea,---;diamyl ether, and N,N-dibutylstearamide might indicate that they would be satisfactory, but actually the compounds are toofinsoluble in water to serve as hydrogen bonding agents,in 5m aqueous solution.

.. Inladdition to being water-soluble a hydrogen bonding donor should not contain .groups which will react with silicic acid to give compounds of the conventional sort when such reaction would 'interfere with hydrogen bonding. The donor preferably should not react with silicic acid to give a precipitate.

As has been indicated, the water-soluble, or- 60 ganic hydrogen bonding donor compounds used according to this invention are preferably selected from the group consisting of ethers, amides, alcohols, and ketones. It is preferred that'the ethershave an oxygen or nitrogen atom in addition to that inthe ether linkage and specifically it is desirable that the ether contain in addition to the ether group a second ether group, a hydroxy group, an amide group, or an ester group. Amides for use with silicic acid sols according to the present invention are preferably N-substituted. Alcohols for use as donors ought to contain more than two carbon atoms. Alcohols are among the weakest of the hydrogen bonding agents but may find value in some instances. Ketonesare not quite as weak as alcoas above discussed. Hydrogen bonding agents for use with silicic acid sols may be characterized by the description that they are non-alkaline, organic, compounds containing at least one electro-negative atom-selected from the group'consistingof nitrogen and oxygen to which is attached no more than (Xl) hydrogen atoms where X is the minimum normal valency of the electro-negative atom, and to which hydrocarbon groups are attached by the other valences, there being in the donor com-o pound a, number of carbon atoms no less than two and atleast twice as large as the number of hydro'gen atoms attached to said electro-negative atom. This description of course omits ureas which are, however, specifically described and claimed in my co-pending application Serial Number 355,606, filed September 6, 1940.

As has been indicated above, ethers are among the preferred hydrogen bonding donor compounds for use with silicic acid sols according to the presentinvention. In addition to containing an ether group these donors may advantageously contain an additional ether group, a hy-, droxy group, an amide group, or an ester group. The presence of these groups appears very beneficial. A number of such groups may be present and there may be used, for instance, polyethersp which contain hydroxyl groups and ester groups. 3 As examples of ethers the following are listed:

Nonaethylene glycol Dimethyl ether of tetraethylene glycol Dimethyl ether of diethylene glycol Hexaethylene glycol Butylether of diethylene glycol Ethyl-ether of diethylene glycol acetate Methyl ether of diethylene glycol (acetate Tetraethylene glycol a v Monoethyl ether of diethylene glycol Triethylene glycol Monoethyl ether of ethylene glycol Monomethyl ether of-ethylene glycol Diethylene glycol N,N'-bis('beta-methoxyethyl) adi N,N-bis(methoxymethyl) urea Polyethylene glycol 'adip'ate Diethyl ether of diethylene glycol Dioxane v I Dioxolane r I Diethyl ether or ethylene glycol Dimethyl ether of ethylene glycol Triethylene glycol dipropionate Glycerol formal Ethylene glycol acetal Sorbitol triacetal N,N-dimethylmethoxyacetamide N-acetylmorpholine N,N'vadipyldimorpholine Dimorpholide ureav Polyethylene oxide g Monomethyl ether of ethylene glycol-ethylene, oxide reaction product j Ethylene glycol ethylene oxide reaction product Glycerol-ethylene oxide reaction product Ethanolformamide-ethylene oxide reaction product r Amidcs are among the preferred hydrogen bonding donors for use with silicic acid sols ac.- cording to thisinvention, Wherea-s oxygen is ,N the donor atom in ethers the nitrogen of amides probably acts as the donor atom." Among the most efiective compounds of this group are the N-substituted amides and the di-substituted pamide I t compounds are preferred. It will be seen that -some of the compounds in the above list of ether: are amides as well and could have been shown below as illustrative of the use of amides. of amides specifically in tanning is covered in my aforementioned applicationserial Number 355,- v 606, filed September-6, r

Examples of amides are, listed below, ureas and other amides being listed separately: I

- Urea: Tetramethylurea Tetraethylurea Urea 'I'hlourea. I 7 N,N-diethyl-N,N'-diethanolurea N,N,N',N '-tetramethyladipamide.

N,N-diinethylacetamide' N,N,N',N'-tetramethylsuccinamide N,N,N' ,N -tetraethylsuccinamide N.N-diethylacetamide N,N,N',N'-tetraethyloxamide N,N diethylformamide N,N-diethylpropionamide R N,N-dimethylformamide N,N-,N',N'-tetraethyloxamide N,N-diethylglyco1amide Y N-ethyl-N -ethanolformamide N-ethyl-Nethanolglycolamide N'- (2,3-dihydroxypropyl) methoxyaoetamide N-butyl-N.-.beta-hydroxyethyllactamide Nethanolethanesulfonamide m p N-isobutylacetamlde N,N-diethanoloxamide V Nl l diethanolsuccinamide N-forniylhe'xamethyleriimine Dieth ylcyanamide v Acetamide 5 Formamide Isopropanol Hexamethylene Pentaglycol 'Succinamide Examples of ketones may be given as follows H 'Ketones Acetonylacetone Formacetoethylm Methyl acetoacetate Diacetone alcohol Examples tr alcoholsjmay be given asfollows": I

Alcohols Diace'tone alcohol nediol' I Pinacol i sl w l Tart-b t c i Tetrahydroxy octane I 1;1,l-trimethylolethane Propanediol i '1 Silicic acid sols fornuse as tanning agents ac- T cording tothe present. invention are preferably a, formed in ;the-;prescnce of a hydrogen bonding donor compound, but if desired the, donor may be added, to a freshly prepared $01. The amount of hydrogen" bonder to use'ina particular case varies considerablywith the results desired and with the particular silicic'acid sol and the par- 1 -ticulartanning procedure chosen. The amount of donor should of course not be so great as to slow the tanning down unduly'and yet it should precise amount'of'a particular agent to use in a particulartanning bath can best be determined bya few simple trials. In general, it maybe indicated that there shouldbe -us'ed an' amountof donor equivalent to from'about 0.1 per 'cent to per cent of the weight *of the tanning solution.

The silicic acid sol tanning agents of this invention may be used for the treatment of any type of skin adapted to be converted into leather or fur. It is to be observed that-the term "skin" is used generically to include heavy skins such as cowhide, lighter'skins such as goatskin and calfskin, and also skins of fur-bearing animals. It will naturally be necessary to adapt the processes a v 2,276,315 be suiiicient to prevent too'r'apida tanning. The Y processes. The time allowed "forbe suiiicient to permit complete penetration-bf the tanning agent, and the pfl-"should-not be-" uraised toorapidly'or there may be puckering" and'apparent over-tannage of the grain surface. I

' perature, say "t'o 30 0.- The exact-amount 1" to the type of skin to be treated. For instance,

goatskin may be more rapidly tanned than calfskin, but those skilled in the art are well aware of such factors and may readily adapt the com-, positions of this invention'to the specific of particular specialized problems.

Skins to be tanned according of this invention may be prepared in any of the manners customary in the art; They may suit ably be unhaired, limed, delimed, and bated and pickled. The skins may if desired be de-pickled.

the processes 1 The tanning can best be effected at room te'mof the tanning solution to-use can best be de- The leather'obtained according to the pre ferred processes of this invention using a silicic "acid sol with a soluble aluminum compoundis white, but it may if desired be treated with white pigment or may be colored during or after the tanning processesbyinclusion of any of the" by the leather in-a reasonable period 'of and they may cause. over-tannage.

common leather dyes and pigments. Leather produced by any of the processes of this-inven- They may also be given a salt liquor treatment to soften them and toput them in a more uniform condition as is the,practice in prior art tanning processes. t

While skins prepared in any manner may be treated according to the" processes 'of' this invention, thepH of the skins is important.- As will be observed 'below. the tanning is effected at certain conditions of acidity and" the skins ning solution may suitably be controlled to compensate'for variance of the pH of the skins vfrom that of the tanning solution. It may sometimes be found advantageous to adapt the processes for preparing the skins so that a prepared for taming is approximately at the pH of the tanning solution.

-Too rapid reaction of the tanning agent with the skins in the first stages of theprocessshould be avoided in order to prevent surface hardening of the skin. The amount of hydrogen bonding donor used should, as indicated above, be such as to assist in preventing toofirapid a tanning and the initial pH of the solution should not be too high. The initial pH of the solution will ordinarily be from about 1.5 to 3.0. More,

specifically, it will be foundpreferable to use a pH of about 2.5. If the initial pH ismuch above 3.5 the tanning will proceed too rapidly in the initial stages though it will be understood that 'the pHto be selected in any. para used in conjunction with vegetable tanning ma- "as may tend to change these conditions. The tantion may similarly be colored and dyed in accordance with'the practices known in the art. Silicic acid sol tanning agents, including a hydrogen bonding donor according to the present invention may if desired be used with other" tanning agents or tanning assistants heretofore known to the art. They may, for instance, be

teriais such as quebracho extract, with chrome tans, or with synthetic tanning agents such :as those containing sulfonic acid groups. The silica sols may be used to begin tanning and other tan ning agents used to finish off the leather. or

alternatively, the other tans may be used to begin the tanning and the silicic acid sols used for finishing. when other tanning agents are used simultaneously with the silicic acid sols due consideration must of course be given to the acidity of the resulting solution so that it will fall within theranges heretofore mentioned.

As has been suggested in the fo egoing, tne t 'ii ning solutions may contain any of the various additions suggested by the art as beneficial in silicic acid sols. Suitable dispersing or stabilizing agents may be used as desired and in generalthe practicular instance will depend upon the specific co'nditions of the tannage. If too low a pH is used combination of silica with the skin is too slow and, additionally, excessive acidity may deleteri'l ously aifect the skin. I

After satisfactory penetration *has been obtained the pH of the solution'is raised by the addition of any suitable alkali. Raising the pH accelerates the tanningaction. The pH should at this point be raised to about 3.5 to'4.5. Sodium carbonate or other alkali metaltor ammonium hydroxides or carbonates or organic bases-such as ethanolamines may be used to raise the pH.

The considerations asto thetype of apparatus to usethe time to be allowed for tanning,

the temperatures of tanning,-and the amounts of tanning agent are substantially those known to the art in connection with prior art tanning -nent. t Leather produced according to the processes of tices in the art with respect tosilica sols may,

advantageously be employed in specific instances. The tanning agents of this invention may be oflered to the trade as sols containing a hydrogen bonding donor compound and additionally conf taining metals and other assistantsas desired;-

If it is likely that the tanning agents will not be used for quite awhile it may be found preferable to'sell two components, one, for instance/con taining sodium silicate and any suitably modifying agents which do not lower the pH to a point so low that a sol is formed, and; a second-component containing a hydrogen bonding donor compound and anacidic materialin such amount that when the two' components are mixed a sol .of substantially the character desired will be compound is to be used itwill ordinarily be found preferable to this invention may be subjected to any of the various kinds of finishing treatments customarily nni ng met.-

include it in' the second compo- V used. The leather, if desired, may be suitably filled or given any of the numerous surface treatments customary in the art.

My invention will be better understood by reference to the following illustrative examples:

. Erma: 1-

A tanning solution was prepared according to the present invention by making up the A andB solutions as tabulated and adding solution B to solution A with agitation. The tanning solution was clear and had a pH value of 2.4 In the following parts are given as parts by weight:

Ina tanning drum there was placed 50 parts (drained pickled weight) of goatskins which had been unhaired, delimed, and pickled in customary manner. The skins were agitated .for ten minutes with 50 parts of a 10 per cent solution of sodium chloride and then the tanning solution prepared as above was added. The skins were agitated and were neutralized to pH 4.3 during the course of one hour and forty minutes by repeated additions of a 10 per cent solution of sodium bicarbonate. The. skins were removed from the drum and piled down over night.

The skins were returned to the drum containing the tanning solution and neutralization continued to pH 5.8; The skins were then washed and fatliquored and after drying, stak- 1:3.25 10.12 Water V 22.00

Leather of good quality was produced.

- Exmu 4. A tanning procedure was followed as in Example 1 using a tanningsolutionmade up with the following: i SolutionA 4 sulfuric acid 7.12 Water 21.00 Nonaetlrvlene glycdl;;. 0.40 Aluminum sulfate 9.80

' Solution 8 Parts 42.5 B. sodium silicate (Nazozsioz) ratio 1:3.251' 10.12 Water 22.00

Water ing, and tacking the resulting leather was smooth, white, full, and supple.

EXAMPLE 2 A tanning procedure was used as shown in Example 1 but using instead of the sol of Ex- A tanning procedure was followed as in Example 1 using a tanning solution made up with the following:

Solution A Parts 20% sulfuric acid 7.12 Water 21.00 Tertiary butyl alcohol. 1.00

Aluminum sulfate 9.80 7. In a tanning process, the step comprising SolutionB 42.5"B. sodium silicate mm :fsion ratio Leather of good quality was produced.

Exam 5 A tanning procedure was followed as in Example 1 using a tanning solution made up with I the following:

425 B. sodium silicate (NazOzslOz) ratio 22.00 The leather produced was of good character.

While I have shown certain illustrative tanning compositions and processes it will be understood that without departing from the spirit of this invention one skilled in the art may readily use yarious water-soluble hydrogen bonding donor compounds with silicic acid sols in a wide varietyof ways.

- This application is a continuation-in-part of application Serial Number 355,605, filed Septemher 6, 1940.

I claim:

1. A tanning composition comprising a silicic acid $01 and a'water-soluble, organic hydrogen bonding donor compound.

2. A tanning composition comprising a silicic acid sol, a water-soluble hydrogen bonding donor compound, and a soluble aluminum compound.

3. A tanning composition comprisinga silicic acid s01, a water-soluble ether, and aluminum 6 P 2,s7o,s1s; subjecting. a skinto} Mumnin: .solutioncomprising a silicic acid sol, avatar-soluble hydrogen bonding, donor compound, and a soluble iglu-c; acid sol and a water-soluble kehone.

minumcompound. I: 8. In a tanning process. the step comprising subjecting a. skin to altanninzlsolution comprising a silicic acid sol, aywater-solubie ether, and

aluminum sul!ate.- I

9. In a tanning process, ialie sun-comprising.

subjecting a skin to s tsnningsolution comprishydrogen bonding donor compound.

10. composition" -scld :01 and a water-soluble alcohol;- 1

11. Atanning composition comprising a silicic m in silicic acid soland a water-:soluble ketone. 

